This application claims priority of German Application No. 102 05 189.5, filed Feb. 6, 2002, the complete disclosure of which is hereby incorporated by reference.
a) Field of the Invention
The invention is directed to a method for generating extreme ultraviolet (EUV) radiation based on a radiation-emitting plasma, particularly for generating EUV radiation with a wavelength around 13 nm. It is preferably applied in EUV lithography for semiconductor chip fabrication.
b) Description of the Prior Art
Increasingly smaller structures in semiconductor technology are testing the physical boundaries of currently available exposure methods for producing semiconductor structures. This is caused by the structures on the order of magnitude of the wavelength of the light used for the lithography methods that are employed. Smaller structure sizes accordingly require the use of light of increasingly smaller wavelength. Therefore, EUV lithography is one of the most promising developments for future lithography methods.
Beam guiding and beam shaping in EUV radiation are made possible by multilayer mirrors. Reflection optics (multilayer mirror optics) by which EUV radiation can be bundled and guided can be realized by means of these multilayer mirrors. Reflection optics of this type which usually contain molybdenum and silicon have their greatest reflectivity (about 70%) in a wavelength range which can be adjusted depending on the thickness of the applied layers and depending on the angle of incidence of the radiation, but which should always be on the long-wave side of the L-absorption edge (12.4 nm) of silicon, since silicon absorbs only slightly in this case.
Currently known EUV sources usually work with xenon. With laser-induced plasmas, the laser radiation is focused on xenon at high intensities, whereas, with gas-discharge sources, xenon is used in the discharge chamber as a work gas either in a mixture with another gas or in pure form. Xenon, which is gaseous under normal conditions, can be used without further processing in gas-discharge sources but is less suited as a target material for laser-induced plasmas because of its low density. The condensation of xenon that is attempted for this reason poses a technological challenge because the temperature range of the melting phase is very small (about xe2x88x92108xc2x0 C. to xe2x88x92111xc2x0 C.) and consequently an elaborate cooling installation with very good temperature regulation is required for handling xenon. Further, xenon has the disadvantage of very high cost (about 10 euros per liter of gas), which represents an added disadvantage to its use as a liquid (with correspondingly high consumption).
The spectral distribution of the light emission of xenon (ionization factor of 8 to 12, 4d-4f transitions) typically exhibits a maximum in the wavelength range between 10.5 nm and 11.0 nm, that is, on the short-wave side of the silicon line. The position of the maximum can be shifted slightly by varying the plasma parameters of density and temperature, but only by a few tenths of a nanometer (10xe2x88x9210 m) in practice, as is described by G. Schriever et al. (G. Schriever, K. Bergmann, R. Lebert, xe2x80x9cExtreme ultraviolet emission of laser-produced plasmas using a cryogenic xenon targetxe2x80x9d, J. Vac. Sci. Technol. B 17 (5) (1999), 2058-2060). This means that the emission characteristic of xenon is relatively poorly adapted to the reflection characteristic of the multilayer mirrors described above and large proportions of radiation are accordingly absorbed within the silicon layers of the optics.
The emission characteristic of xenon has already been investigated for many years in gas discharge sources because it is easily managed as a gaseous material. This led to auxiliary theoretical observations in which the emission is described by Hartree-Fock calculations (e.g., J. Blackburn, P. K. Carroll, J. Costello, G. O""Sullivan, xe2x80x9cSpectra of Xe VII, VIII, and IX in the extreme ultraviolet: 4d-mp, nf transitionsxe2x80x9d, J. Opt. Soc. Am. 73, No. 10 (1983) 1325-1329). The extensive investigations in the past with experimental and theoretical results have made xenon a universal and very well-known target material for EUV sources. The radiation outputs achieved for the investigated applications were sufficient for these applications, but are too low for EUV lithography in connection with beam-shaping optics.
It is known from another publication by W. T. Silfvast et al. (xe2x80x9cLaser-produced plasmas for soft x-ray projection lithographyxe2x80x9d, J. Vac. Sci. Technol. B 10 (6) (1992), 3126-3133) that tin is a broadband emitter in the wavelength range between 13.0 nm and 13.5 nm. In this connection, tin was used as a solid target for laser-induced plasmas. The greatest disadvantage of tin is its extensive debris emission. It is particularly disadvantageous that this material can be removed from contaminated surfaces only with difficulty due to the high boiling temperatures of tin (approximately 2602xc2x0 C.).
It is the primary object of the invention to find a novel possibility for generating extreme ultraviolet radiation based on a radiation-emitting plasma in which the emission output of the EUV source is increased to the wavelength range above the L-absorption edge of silicon without substantially increasing the technical and monetary expenditure for plasma generation.
According to the invention, this object is met in a method for generating broadband extreme ultraviolet radiation through emission of radiation from plasma under vacuum conditions in that the plasma is generated using at least one element from V to VII in the p-block of the fifth period of the periodic table of elements.
The plasma is advantageously generated with the participation of iodine or iodine compounds.
Further, the plasma is advisably generated with the participation of tellurium or tellurium compounds or with the use of antimony or antimony compounds.
The plasma can advantageously be generated with the participation of chemical compounds of iodine with tellurium or antimony, particularly tellurium-iodide and antimony-iodide.
In order to achieve a particularly intensive radiation yield of about 13 nm, it has proven advantageous to generate the plasma from chemical compounds of iodine with lithium or fluorine, particularly LiI and IF7 or IF5. This results in the superposition of the broadband iodine emission and line emissions of lithium and fluorine.
The suggested materials for plasma generation are advantageously suitable for gas discharge-based EUV sources in that they are evaporated and introduced into the evacuated discharge chamber as work gas.
On the other hand, the materials for plasma generation are equally advantageously suited to laser-based EUV sources in that they are introduced as target material for the excitation radiation by means of laser radiation. The material can be introduced as liquid target material as well as in a solid aggregate state.
The invention is based on the idea that xenon, the material that is usually used for generating EUV radiation, does not actually have the optimal radiation characteristic for the multilayer reflection optics made of molybdenum and silicon that are currently available for transmission of EUV radiation because the reflectivity of the optics below the L-absorption edge of silicon (about 12.4 nm) is considerably limited. The large quantity of optical systems in a lithography stepper or lithography scanner (approximately 10 in a series-produced device) results in excessive radiation loss. In addition, particularly for laser-induced plasma, the cost of condensing xenon when used as target material, together with the price of xenon which is already high by itself, represents a substantial cost factor.
On the other hand, tests with metallic target material comprising tin which are known in laser-induced plasma generation are unsuitable for long-lasting EUV sources because of excessive debris and the resulting limitations.
Surprisingly, however, it has been shown that the elements iodine, tellurium or antimony (and chemical compounds thereof) which were discovered because of their electronic similarity to xenon have considerably better characteristics than tin with respect to debris. While the debris is not negligible, it is much easier to remove due to much lower melting points and boiling points in that the contamination of surfaces is eliminated by evaporation at permissible heating temperatures. In addition, the low boiling points also facilitate use in gas discharge sources for generating EUV.
The lower atomic numbers of the suggested elements compared to xenon result in an emission of photons with less photon energy and a greater wavelength, with all other electronic preconditions (ionization coefficient) and intraion transitions remaining the same. Therefore, the elements are better suited than xenon for the emission of radiation at a wavelength of around 13 nm.
The method according to the invention makes it possible to generate EUV radiation based on a radiation-emitting plasma in which the emission output of the EUV source in the wavelength range is increased above the L-absorption edge of silicon without a substantial increase in technical and monetary expenditure compared to plasma generation by means of xenon. In particular, the method achieves a better matching of the emission of an EUV source to multilayer reflection optics comprising molybdenum and silicon. Further, the invention substantially reduces the cost of material for plasma generation compared to xenon.
The invention will be described more fully in the following with reference to embodiment